The invention relates to a ceramic filter element for tangential flow filtration of liquids and gases where the filter element has an elongated support body of a porous ceramic material, at least two-coaxial channels running through the support body with at least one ceramic membrane on the surface of the channels.
Ceramic membrane filters have proven their worth especially in hydroeconomy and in the beverage industry for the filtration of beer, wine and fruit juices in which tangential flow (cross-flow) filtration enjoys preference in continuous processes. With this application, called "dynamic filtration," suspensions can be filtered without clogging the membrane. The liquid to be filtered is not forced directly through the membrane but made to flow past the surface of the membrane with an appropriately high velocity of flow ranging from 2 to 7 meters per second, while only a part of the liquid stream passes through the membrane as filtrate (permeate). The dogging of the membrane, i.e., the formation of a filter cake on the membrane, is prevented by the fact that, due to its high velocity of flow and the microturbulences occurring at the membrane surface, the suspension constantly flushes away the particles held on the membrane surface.
Ceramic filters for tangential flow filtration, such as those known, for example, from DE-A-35 19 620, have combined layers of different porosity and defined pore size. The thin top membrane layer performs the separating function, and the coarse ceramic layer beneath it serves as a supporting layer. Filter elements are said therein to be especially effective which in general have an elongated, cylindrical form of the supporting layer structure, referred to hereinafter as the "support body", with a plurality of bores extending through the support body and with a very thin ceramic membrane applied to its surface as the separating means.
Filter elements with a plurality of coaxial cylindrical channels through which only unfiltrate flows have proven their worth in the beverage industry. The filtrate penetrating through the membrane surface of the channels flows through the support body under pressure and exits at its circumferential surface. The disadvantage of these known multichannel filter elements lies in the low rate of flow of the filtrate through them. Particularly the inner channels contribute little to the performance of the filter, although they have a comparatively large portion of the membrane surface. Experiments have shown that the support body itself has a non-negligible resistance. A test of this kind is given as an example further below.